THERMODYNAMIC PROPERTIES FOR SOLUTION OF HYDROGEN IN PALLADIUM-BASED BINARY-ALLOYS

The thermodynamic parameters for solution of hydrogen at infinite dilution and the (alpha+beta) plateau thermodynamic properties for hydrogen absorption at moderate temperatures for a series of Pd-based f.c.c. binary alloys with relatively low solute metal content have been correlated from the viewpoint of whether the alloy lattice is expanded or contracted to parent Pd lattice. Newly determined data for Pd-Zn, Pd-Sb, Pd-Bi, Pd-Cr, Pd-Mo and Pd-Mn alloys are included. When alloys with solute metals in the same groups are compared, the lattice parameters of Pd binary alloys with the 4 d and 5 d transition elements are, in general, larger than those with the 3 d elements. The relative partial molar enthalpy, Delta H-H(O), at infinite dilution becomes more exothermic with an increase of lattice parameter for the lattice-expanded alloys with the exception of Pd-Pt alloys. Conversely, for contracted alloys, with the exceptions of Pd-Li, Pd-Zn, Pd-Al and low Ti content Pd-Ti alloys, the enthalpy becomes less exothermic with increasing lattice contraction. The dilute phase solubility at the same temperature as reflected by the relative chemical potential of dissolved hydrogen at infinite dilution, Delta mu(H)(O), increases with increasing lattice expansion for the expanded alloys, with the exceptions of Pd-Pt and Pd-Nb(Ta), whereas for the contracted alloys, the solubility decreases with increasing lattice contraction, with the exception of the Pd-Li alloys. The standard free energy change, Delta G(plat)(O), for beta-hydride formation in the expanded alloys decreases with increase of lattice expansion with the exceptions of the Pd-Pt, Pd-Zr, Pd-Sb, Pd-Nb(Ta) and Pd-Mn alloys. For the contracted alloys, the P-hydride becomes less stable with increasing lattice contraction except for the Pd-Li alloys. The isobaric hydrogen solubilities in Pd alloys at relatively high pressures may be attributed to the influence of the solute metal atoms on the Pd band structure and to the exclusion of H from site occupation due to solute atoms in the nearest neighbor shells; both of these effects are independent of the lattice expansion and contraction.